Photovoltaic (PV) modules convert incoming sunlight into electrical energy. The accumulation of dust, particles, and dirt on the surface of PV modules, known as soiling, negatively impacts the solar energy industry. Soiling causes a reduction in the sunlight that reaches PV modules, causing a reduction in electrical power production. The reduction in electrical power production because of soiling accumulation is known as soiling loss. Soiling losses may cause a 50% reduction in electrical power generation, or even higher, in particularly adverse conditions. In the worst conditions soiling may cause power losses as high as 70%. Maintenance to the PV modules is required to remove the soiling and restore performance, but cleaning PV modules may be expensive. Identifying soiling is essential to planning an effective maintenance schedule which maximizes the electrical power production of PV modules and limits operating costs.
Soiling losses are due to the drop in optical transmittance, because soiling absorbs part of the incoming sunlight and increases the portion of reflected light, limiting the amount of light available for conversion to electrical energy. Moreover, soiling changes the spectrum of the transmitted light, (e.g., causing larger transmittance drops in the blue region) meaning that the various PV technologies are differently affected by soiling, depending on their bandgaps. The impact of soiling may depend on the spectral response of the specific PV material under investigation.
Some PV modules absorb the entire light spectrum (i.e., the entire global irradiance), while others absorb only a portion of the light spectrum. Concentrator PV (CPV) modules convert the direct component of global irradiance to electrical energy. Soiling produces a higher impact on the direct transmission of the irradiance CPV systems show a higher and different dependence on soiling than conventional PVs. Soiling produces a higher attenuation in the blue region, and therefore causes a red-shift of the incident spectral distribution.